When u use a p2p connection there is no way in which u can share files when offline.
The basic principle in which p2p works is by providing peer to peer connection i.e both users are on the same level of connection so since there is a direct link in between and no space for buffer storage of the files there is no way the person downloading file from u can keep downloading it if u r offline.
As for ur doubt tat u may have to pay for wat u download .Well wat ever u download is being share and it is free but the catch is if u r downloading copyright stuff there its illegal so music programs etc are a big no no.
Then the q comes wat can u download.Well some ppl do share stuff which they have made like music or digital animation etc they are legal .But its again upto u can use a p2p network as u want it.
Edonkey is the best p2p network around now.U can access it usin clients like edonkey emule or shareaza (which supports other n/w other than edonkey)
There is also a new client call kdrive which promises to keep ur identity hidden dunn kno how it is as its pretty new.It also helps in creating groups of users who can share files within themselves.
Also there is mldonkey which can be use in linux.
U can get info abot all the new stuff in p2p world from:
www.zeropaid.com